Dimerization constant

The aniline-zinc porphyrin interaction has also been exploited to form dimers. Hunter (60) reported the dimerization of porphyrins functionalized at one meso position with ortho or meta aniline groups (47, 48, Fig. 15). Both compounds showed concentration-dependent H NMR spectra with large upfield shifts for the aniline protons. The dimerization constants are 160 and 1080 M-1 respectively for 47 and 48, and these values are an order of magnitude higher than the association constants of simple reference complexes (K — 10 and 130 M 1 respectively), which is indicative of cooperative self-assembly. The complexa-tion-induced changes in chemical shift were used to obtain three-dimensional structures of the dimers. 

At high acid concentration, particularly in perchloric acid medium, molybdenum(VI) shows a strong tendency to dimerize (68-71). The value for the dimerization constant of the cation [Mo02(OH)(H20)3]+ in 3.0 M Na(H)C104 medium has been determined, K = 97 15 (71). 

On the basis of such an analysis the dimerization constant of Cp2TiCl was determined to be 3 x 103 M This implies that for Ti(III) concentrations above 1 mM the dimer becomes the dominant species. However, the Zn-reduced solutions of the other and more sterically hindered complexes 21-24 are monomeric, the exception being that of 24, since the appearance of the voltammogram remains essentially the same on changing the concentration. In Fig. 2b this is illustrated for the case of 21. 

Furthermore, although the intercepts k kiK/k- ) and the slope (kikjK/k-i) are equally influenced by the dimerization constant K in equation 28, this does not imply that they should show the same effect on changing the solvent. According to the dimer mechanism , it could be expected that the base catalysed decomposition of the transition state SB2, measured by Ag, should be more depressed by small additions of protic solvents than the spontaneous decomposition measured by Ag. Indeed, the overwhelming evidence on the classical base catalysis by amines shows that usually Ag is more important in aprotic than in protic solvents1. 

Figure 18 Working curve to obtain the dimerization constant from the current ratio iprlip/...

Insulin Lispro was the first recombinant fast-acting insulin analogue to gain marketing approval (Table 8.3). It displays an amino acid sequence identical to native human insulin, with one alteration — an inversion of the natural proline lysine sequence found at positions 28 and 29 of the insulin jS-chain. This simple alteration significantly decreased the propensity of individual insulin molecules to self-associate when stored at therapeutic dose concentrations. The dimerization constant for Insulin Lispro is 300 times lower than that exhibited by unmodified human insulin. Structurally, this appears to occur as the change in sequence disrupts the formation of inter-chain hydrophobic interactions critical to self-association. 

Analysis of the pyrene-labeled homoduplex 5 5 (Eig. 9.3b) by NMR, mass spectrometry, and TLC suggested that 5 5 had a stability similar to that of 3 4. NOESY spectra revealed cross-strand NOEs consistent with the formation of the self-dimer 5 5 (Zeng et al. 2003). Based on a fluorescence method described in the literature (Sontjens et al. 2000), the dimerization constant of the pyrene-labeled duplex 5 5 was found to be (6.77 + 4.12) x 10 M. The studies on duplexes 3 4 and 5 5 clearly demonstrated that the stabilities of our duplexes are indeed only determined by the number of intermolecular H bonds, and both hetero- and homoduplexes can be easily designed and constructed. 

Tabus 1. Dimerization constants (c /Cy.) in water (g, mole/litre)... 

The continuous curve shown in Fig. 2 was calculated from Eq. (5) by use of the following values for the two parameters K = 11.2 mole-1 liters, k — 41.9 ml. (at 515 mm.) min.-1 mole-1. This dimerization constant is in agreement with the approximate value established by separate ebulliometric experiments. It is clear that within experimental error the data can be fit by an expression of the form of Eq. (5) i.e., the data are consistent with the hypothesis that the active catalyst is a dimer of cuprous acetate. 

Discussion The first-order dependence on the total concentration of silver or cuprous acetate for the hydrogenation rate can be interpreted on the basis either that the catalytically active species is a monomer or that it is a dimer, provided, in the latter case, that the dimerization constant is so high that substantially all the catalyst molecules are present as dimers. However, molecular-weight measurements showed that over the concentration range in which kinetic studies were made, dimerization of cuprous or silver acetate was small. This means that in this system it is the monomer of silver or cuprous acetate which is catalytically active, in marked contrast to the cuprous acetate-quinoline system (see above). 

The existence of the dimeric ("2 2") complexes in solution has been detected for complexes of Cem with both ai- and w2-[P2Wi7O6i]10 (Sadakane et at, 2001, 2002). Based upon concentration-dependent 31P-NMR data the dimerization constant for the ai system is 20 4 M 4 at 22 °C. The corresponding value for the... 

The monophthalocyanines 106 and 107 show a weak aggregation tendency in chloroform. The latter has a self-dimerization constant of 1,175 M-1. By contrast, the donor-acceptor bis(phthalocyanine) 99 exhibits a much stronger aggregation tendency with a dimerization constant of 1.1 x 106 M-1 in chloroform. It is believed that in addition to the hydrophobic effect, the two phthalocyanine halves of compound 99 may be considered as donor and acceptor subunits that interact with each other. As revealed by electron microscopy, 99 forms one-dimensional nanoaggregates through intermolecular interactions between its complementary donor and acceptor phthalocyanine units as shown in Fig. 8. The dimerization constant of 99 is about one order of magnitude lower than that observed for the hetero-dimerization of 106 and 107, which may be due to the cyclophane step that hinders the formation of columnar aggregates of double phthalocyanine dimer. 

The most commonly studied pyrrolopyridine is compound (1). This compound is capable of selfassociation, a phenomenon that has led to much investigation, especially by NMR techniques. NMR data in carbon tetrachloride and benzene show that the chemical shift of the H-l proton moves downfield as the concentration of solute is increased. This type of shift is typical for a proton that undergoes hydrogen bond formation. From the NMR data, the dimerization constant for 1H-pyrrolo[2,3-/>]pyridine (1) has been determined to be 18.9 + 10.3 mol l l <8IJPC3l8l>. 

Meijer et al. reported the self-aggregation behavior of a molecule in chloroform (Fig. 11.8) containing two units of 2-ureido-4-pyrimidone linked through a spacer (which self-associates in the DDAA-AADD pattern strongly with a dimerization constant >106 m ) [36], This compound formed viscous solutions in chloroform, and the viscosity observed was dependent on the concentration and temperature. 

As discussed in Sect. 2.5, the ability of the host and guest to self-associate must be established prior to any complexation studies. Carboxylic acids 33, 34, 47, and 49 showed no tendency to aggregate in chloroform [59]. In contrast, the lH NMR of carboxylic acid 31 contained several broad resonances that were consistent with a zwitterionic structure and/or an aggregate [56]. For this reason, complexation studies have not been carried out with the highly twisted molecular tweezers 31 or 32. For the guests used in these studies, dimerization constants were known and indicated that self-association would be negligible under the concentrations used. 

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